1. Field of the Invention
The present invention concerns the technical field of mineral fillers, in particular for paper-making applications, and their improvement by suitable treatments in order to improve either the manufacturing process of the paper sheet or its properties.
2. Description of the Background
Such fillers are well known to a specialist in the field and we can quote for example natural calcium carbonate, synthetic or “precipitated” calcium carbonate (“PCC”), and various fillers such as dolomite, mixed fillers based on different carbonates of metals such as calcium joined with magnesium and similar, various fillers such as talc and similar, and mixtures of these fillers, for example mixtures of talc and calcium carbonate, calcium carbonate and kaolin, or mixtures of natural calcium carbonate with aluminium hydroxide, mica or synthetic or natural fibres.
It will also be useful to give details of the process of manufacture of a sheet of paper, board or similar. A specialist in the field will know that a paste (“pulp”) is made, essentially comprising fibres (cellulose fibres of natural origin such as from resinous or deciduous wood or of synthetic origin or a mixture of both), a filler as defined above and a suitable proportion of water.
A thick paste or “thick stock” is made and then diluted with water to produce a diluted paste or “thin stock”. The pulp receives various additives, such as certain polymers, in order to improve the conditions of flocculation and therefore of “forming” of the sheet, of water retention and of water drainage under the wire. The aqueous medium containing a fraction of the initial filler, which is drained under vacuum under the wire is known as “white liquor”. The sheet then undergoes various processes, including an important operation known as coating. It is known that during this coating operation there occur losses of coating color and of coated paper. This coated paper is recycled as a mass filler and is known as “coating broke”.